Conventionally airborne potential field surveys such as gravity surveys are flown on a grid pattern. The grid is defined by orthogonal sets of parallel lines (flight paths) on a two-dimensional surface which is draped over the underlying terrain. The draped surface satisfies a minimum height constraint (defined by the closest the aircraft is permitted to fly to the ground) and by a constraint on the maximum rate of climb/descent of the aircraft, typically around three percent. This approach suffices for flat terrain but for hilly or mountainous terrain the surface on which the aircraft flies can vary by as much as two or three kilometres from, say, the bottom of an underlying valley to the top of the mountains/survey area so another approach is needed.
It is useful to be able to collect potential field data, in particular gravity data, from close to the ground i.e. a low height. In a gravity survey nearby mass provides both high and low (spatial) frequency data, whereas the influence of a deeper mass is seen primarily at lower frequencies only. When looking for underlying anomalies the intervening mass has a dominating effect and to provide an accurate representation of deep features a good representation of surface features is desirable so as to be able to subtract-off particularly the higher frequencies (which dominate the power spectrum). For example, a signal with wavelength λ falls off with height z as exp(−kz) where k=2π/λ from which it can be estimated that a signal component of wavelength 200 metres from a mass at a depth of 100 metre has fallen to approximately 1/20 of its initial value at the earth's surface (and is progressively further attenuated with increasing height), whereas it can be seen that longer wavelengths are much less attenuated. Generally the size and position of a survey is broadly chosen according to a wavelength scale corresponding to a signature expected given the target's size and depth.
It will be appreciated from the foregoing discussion that it is generally desirable to be able to perform a flight survey at a low height, but in practice the aircraft's limitations and gridded flight plan can impose significant restrictions. The gridded flight plan is necessary because conventional techniques for processing gravity survey data rely on a constant height assumption. Broadly, the assumption is that for a deep source the aircraft's height is approximately constant, the error from this assumption supposedly being only a small correction. Furthermore conventional gravity survey data processing techniques rely on regularly spaced data points, normally a power of two, so that a (fast) Fourier transform can be applied, this defining the requirement for orthogonal sets of parallel flight paths. The flight paths are required to be in a common surface since existing techniques assume, for example, that where two paths cross they cross at the same height. A further problem with existing flight paths arises when the surveyed area is not precisely rectangular, for example because of the local terrain. In this case in order to be able to apply the conventional techniques the data points are “padded”, for example by interpolation or extrapolation to generate a regular set of data points over a rectangular area. The wavelengths (or more particularly, wavenumbers) used for the Fourier analysis are then determined by the maximum x and y (length and breadth) dimensions of the now padded rectangular area.
In view of the above drawbacks of conventional techniques, improved potential field survey data processing techniques and survey flight patterns are desirable.